Over half of all deaths in non-ischemic heart failure (HF) are due to ventricular arrhythmias, primarily ventricular tachycardia (VT) degenerating to ventricular fibrillation (VF). In non-ischemic HF, VT can initiate by non- reentrant (focal) mechanisms likely due to delayed after depolarizations (DADs). The heterogeneous HF substate may then allow focal activity to convert to reentrant VT/VF. Thus, lethal arrhythmias in HF likely require a focal triggr for initiation and a vulnerable substrate that promotes reentry. HF electrophysiological remodeling can promote DADs and cause pro-arrhythmic changes to the substrate. -adrenergic receptor (-AR) stimulation can exacerbate DADs and further perturb the abnormal ionic currents and Ca2+ transients (CaT) found in HF. Moreover, cardiac sympathetic nerve remodeling in HF may lead to localized -AR stimulation and spatially heterogeneous effects. Electrophysiological and sympathetic remodeling has been individually linked to arrhythmia in HF. However, the interplay between local -AR stimulation and altered HF electrophysiology in arrhythmogenesis has not been explored. The overall objective of this proposal is to systematically determine the role of local -AR stimulation in producing the trigger and substrate for ventricular arrhythmias and how electrophysiological remodeling in non-ischemic HF exacerbates these effects. Our over-arching hypothesis is that local -AR stimulation causes 1) spatiotemporal synchronization of DADs across many cells to provide focal triggers; and 2) local electrophysiological heterogeneity to produce the substrate for reentrant VT/VF. We further hypothesize that electrophysiological remodeling in HF exacerbates the pro-arrhythmic effects of local -AR stimulation, both in generating triggers and in modifying the substrate. To address these hypotheses, dual optical mapping of Vm and Ca2+ will be performed on isolated rabbit hearts while administering local norepinephrine (NE). Aim 1 will focus on the mechanisms by which local -AR stimulation triggers focal arrhythmia in healthy rabbit hearts. We will then systematically test the effects of HF- associated electrophysiological remodeling on the propensity to focal activity by pharmacologically mimicking key HF phenotypes. Aim 2 focuses on the role of local -AR stimulation in contributing to the substrate for reentry via dispersion o action potential and CaT properties and development of alternans. We will also test the effects of key HF-associated electrophysiological mechanisms in contributing to reentry during local -AR stimulation. In Aim 3, we will determine the arrhythmogenic role of localized -AR stimulation in a rabbit model of non-ischemic HF by applying exogenous as well as invoking endogenous sympathetic stimulation followed by a quantitative assessment of neurochemistry in HF. We will then assess the therapeutic potential of reversing key HF phenotype(s) with newly proposed anti-arrhythmic strategies. Overall, the results of this project will define the firt mechanistic link between sympathetic dysfunction and ventricular arrhythmias in non-ischemic HF and will greatly advance our long-term goal of predicting and preventing sudden cardiac death in HF.